Bandgap voltage generating circuits are well known in the art. See for example U.S. Pat. No. 6,943,617. Referring to FIG. 1 there is shown a bandgap voltage generating circuit 10 of the prior art. The circuit 10 comprises two parallel current paths, marked as I1 and I2. The current in the path I2 is I2=(Vbe1−Vbe2)/R0=dVbe/R0 (where Vbe1 is the voltage across the base-emitter of the bipolar transistor 12 in current path I1 and Vbe2 is the voltage across the base-emitter of the bipolar transistor 14 of current path I2). dVbe=VT*ln (N), where VT is thermal voltage k*T/q, k=Boltzmann constant, q=electron charge; hence is proportional to absolute temperature (PTAT). Vbe is complementary (or negative) to absolute temperature (CTAT). The output bandgap voltage Vbg=(R1/R0) dVbe+Vbe3 (where Vbe3 is the voltage across the base-emitter of the bipolar transistor 16 in current path I3). The size of the emitter of the bipolar transistor 12 and the bipolar transistor 16 are substantially the same, while the size of the emitter of the bipolar transistor 14 is approximately N times the size of the emitter of the bipolar transistor 12. In general, the disadvantage of the circuit 10 is that the minimum bandgap voltage is high, (on the order of >2 volts).
Referring to FIG. 2 there is shown another bandgap voltage generating circuit 20 of the prior art. The circuit 20 is similar to the circuit 10 shown in FIG. 1 except with the addition of a charge pump as shown. However, the result is similar to the circuit 10 shown in FIG. 1 in that the minimum bandgap voltage is on the order of >2 volts.
Referring to FIG. 3 there is shown yet another bandgap voltage generating circuit 30 of the prior art. The circuit 30 comprises an operational amplifier 32 with two inputs and one output. The operational amplifier 32 receives inputs from a current mirror (34a & 34b). The output of the operational amplifier 32 is used to control a PMOS transistor pair 36 (which is equivalent to one PMOS transistor, circuit wise) connected in series with a resistor 38, with the output of the bandgap voltage taken from the connection of the PMOS transistor pair 36 with the resistor 38. Although the output of the bandgap voltage can be as low as 1.0 volts, the circuit 30 requires multiple precise circuits resulting in potential mismatches.
Referring to FIG. 4 there is shown yet another bandgap voltage generating circuit 40 of the prior art. The circuit 40 comprises an operational amplifier 42 with two inputs and one output. One of the input is taken from a resistor divide circuit (comprising resistors R1 and R2), while the other is from a parallel circuit. The output is used to control the current path through the two circuits. The output of the bandgap voltage is on the order of 1.25 volts.
As more and more electronic devices become portable and use battery as a source of power, this requires the bandgap circuit to have low power consumption as well as being able to generate a low voltage. Hence there is a need for a low voltage, low power bandgap circuit.